How do you eradicate knotweed and replace with a native plant?


I have an area about 200 square feet where we cut, treated and then scraped out knotweed this fall. We know it will be a constant battle to try to completely eradicate for the next few years but would like to replant the area with something native and invasive-ish to help with squeezing out the rhizomes. The area has southern exposure between a house and an outdoor patio with plenty of moisture but not a bog.  The knotweed was a wonderful seasonal screen and the birds used it for cover.  We’d love something on the taller side.  I’m committed to only adding native plants. I understand from my research that something with rhizomes is a good idea. Any help is appreciated!


Marjorie Peronto, Extension Educator

Thank you for your great question.  I do not know of any plant, native or otherwise, that can outcompete Japanese Knotweed.  I am hesitant to recommend that you replant the cleared area with anything else until you know the level of resprouting that may occur.  Even the smallest fragments of Japanese Knotweed rhizomes or stems that were left behind have the potential to resprout.  They would be very difficult to remove once you’ve planted a spreading perennial, herbaceous or woody, in that area.  My suggestion would be to smother the area for at least one year (preferably more) using the method described on pages 7 and 8 of this bulletin Preventing the Spread of Japanese knotweed from the New Hampshire Department of Agriculture.
Once you are confident that you’ve got the knotweed eradicated (mostly), I’m thinking that a planting of our native Northern Bayberry (Morella pensylvanica, formerly called Myrica pensylvanica) may be suitable for that site.  It is an exuberant, rhizomatous, colonizing shrub that thrives in full sun to part shade, evenly moist to dry soils, which will provide a seasonal screen and terrific habitat for birds.  This species is dioecious, with separate male and female plants, so you would want to make sure to purchase both sexes (it is easiest to identify the female plants when they are in fruit).